The prior art discloses batteries which are used in hybrid and electric vehicles and which are referred to as traction batteries since they are used for feeding electric drives.
The basic circuit diagram of an electric traction drive system 10 known from the prior art as is used, for example, in electric and hybrid vehicles or else in stationary applications such as in the rotor blade adjustment of wind turbines, is illustrated in FIG. 1. A battery (traction battery) 11 is connected to a DC voltage intermediate circuit, which is buffered by a capacitor 60. In addition, a pulse-operated inverter or pulse-operated converter 50 is connected to the DC voltage intermediate circuit and provides sinusoidal voltages which are phase-shifted with respect to one another at three outputs via in each case two switchable semiconductor valves 51 and two diodes 52 for operating an electric motor 70. In order to simplify the illustration, only one semiconductor valve and one diode are provided with reference symbols in the drawing. The capacitance of the capacitor 60 is selected to be high enough to stabilize the voltage in the DC voltage intermediate circuit for a period in which one of the switchable semiconductor valves 51 is on. In a practical application such as an electric vehicle, this requires a high capacitance in the mF range.
Three-phase motors are generally used as electric motors (electric machines) 70 in such traction drives 10. These are usually asynchronous motors, permanently regulated synchronous motors or separately excited synchronous motors. In order to feed the electric machine 70, pulse-operated converters 50 are generally used which are generally implemented in the traction region in electric and hybrid vehicles with semiconductor switches 51 which are in the form of insulated-gate bipolar transistors (IGBTs).
The battery 11 illustrated in FIG. 1 comprises a battery module string 12, in which a large number of battery cells 21 are connected in series and optionally additionally in parallel in order to achieve a high output voltage and battery capacity desired for a respective application, wherein, in order to simplify the illustration in the drawing, only one battery cell has been provided with a reference symbol. A charging and isolating device 30 is connected between the positive pole of the battery cells 21 and a positive battery terminal 22. Optionally, an isolating device 40 can additionally be connected between the negative pole of the battery cells and a negative battery terminal 23.
The isolating and charging device 30 and the isolating device 40 each comprise a contactor 31 and 41, respectively, which are provided for disconnecting the battery cells 21 from the battery terminals 22, 23 in order to switch the battery terminals 22, 23 to be free of voltage. Owing to the high DC voltage of the series-connected battery cells 21, there is otherwise a considerable potential risk for maintenance personal and others. In addition, a charging contactor 32 with a charging resistor 33 connected in series with the charging contactor 32 is provided in the charging and isolating device 30. The charging resistor 33 limits a charging current for the capacitor 60 when the battery 11 is connected to the DC voltage intermediate circuit. For this purpose, first the contactor 31 is left open and only the charging contactor 32 is closed. When the voltage at the positive battery terminal 22 reaches the voltage of the battery cells 21, the contactor 31 can be closed and possibly the charging contactor 32 opened. The contactors 31, 41 and the charging contactor 32 represent an increase in the costs for a battery 11 which is not inconsiderable since stringent requirements are placed on the reliability of said contactors and on the currents to be conducted thereby.
If a technical problem occurs in the traction battery of such a traction drive which can either result directly in failure of a battery cell or, on further operation of the battery, in a safety-relevant unsafe state of the battery, the battery is transferred to a safe state by the battery management system. In accordance with the prior art, this state is produced in lithium-ion battery systems by virtue of the battery being disconnected from the DC voltage intermediate circuit by the contactors of the charging and isolating device being opened.
The management system of the traction drive then needs to cope with a situation whereby the battery is no longer available as energy store. Depending on the operating state of the electric motor, the management system now only essentially achieves a situation in which there is no destruction of the pulse-operated inverter or inverter. Destruction of the inverter can be brought about, for example, by an impermissibly severe rise in the voltage of the DC voltage intermediate circuit. As a result, the management system can no longer take into consideration a response of the traction drive which is expedient in terms of driving dynamics or theoretically it is no longer even possible at all to set a response of the traction drive which is expedient in terms of driving dynamics owing to the disconnection of the battery.
As a consequence, the traction drive outputs torques which are undesirable from a driving dynamics point of view or are even impermissible, in particular suddenly occurring, large negative torques. This then needs to be managed in the overall concept of such a traction drive of a vehicle by additional measures, such as mechanical freewheeling, for example. These additional measures are complex and extremely undesirable since they are only required in the case of a technical fault in the battery and are therefore generally never used.
In the earlier patent application by the applicant with the reference DE 10 2010 027 864 A1, a polyphase battery system with a battery with output voltages which are adjustable stepwise is described. The block circuit diagram of a drive system (traction drive) 10 with such a battery 110 is illustrated in FIG. 2. The battery 110 comprises a plurality of battery strings 120, in each of which a plurality of battery modules 130, 140 with battery cells connected in series and/or in parallel are arranged. The battery modules 130, 140 can each be connected in series via so-called coupling units (not illustrated separately) which are associated therewith in each case in an associated battery string 120 with a positive or negative orientation or are bridged. A charging and isolating device 30 is connected in each battery string 120 between the positive pole of an uppermost battery module 130 and a positive battery string terminal 122. Optionally, in addition an isolating device 40 can be connected between the negative pole of a lowermost battery module 140 and a negative battery string terminal 123 in each battery string 120. In particular with a three-phase embodiment, systems with such batteries 110 are also referred to as battery direct inverters (DINV) 110. If a traction drive 10 is implemented using such a battery direct inverter, the traction battery 10 illustrated in FIG. 1 and the pulse-operated inverter 50 are replaced by the battery direct inverter 110.
The possible profile of the output voltage UBS of each phase of the battery direct inverter 110 illustrated in FIG. 2 is illustrated in FIG. 3. The illustrated output voltage UBS is in this case the voltage generated by a battery string 120. FIG. 3 shows the dependence of the output voltage UBS of a battery string 120 on the number k of battery modules 130, 140 connected to form the battery string 120 with a positive or negative orientation. The battery modules 130, 140 connected to form each battery string 120 in this case each have the same module voltage UM. The output voltage UBS of the battery string 120 which is illustrated with a dependence on the connected number k of battery modules 130, 140 is linear and follows the relationship UBS=k·UM for battery modules 130, 140, which have been connected with a positive orientation or a positive polarization to give the associated battery string 120, or the relationship UBS=−k·UM for battery modules 130, 140, which have been connected with a negative orientation or negative polarization to give the associated battery string 120. The relationship 1≦k≦n applies in this case, and n is the maximum number of battery modules 130, 140 which can be connected to form the battery string 120. The maximum output voltage UBS of the battery string 120 can then correspondingly assume the value n·UM. The minimum output voltage UBS of the battery string 120 can then correspondingly assume the value −n·UM.